Hi PEI
What do you mean with loss of cognitive skills?
That pilots forget how to fly? or that their brain doesn't perform so well (like the atrophiated muscles of space station astrounauts)?
When I practice hand flying (no FD no ATHR) regularly I do it quite decently using a small % of my brain. When I haven't practiced for a long time, I have to use a much greater %, and then I make questions like "Have we been cleared to land yet?" or forget to call for the checklist or something.
That is why in my opinion hand flying skills should be practiced and kept in good shape, because it is not about hands, but about brain! And we should be able to have more brain available should things go bad one day.